The present invention relates to an image forming apparatus having a belt and a cleaning member for the belt.
Image forming apparatuses are known in which a recording sheet for forming an image thereon is transported by a belt. In such an Image Forming Apparatus, a cleaning member for the belt is provided to remove developer and paper dust attached to the belt. In terms of the configurations adopted for the cleaning members, the cleaning member is adapted to scrape off the developer by coming into contact with the belt, or by applying a bias to the belt to electrically separate the developer from the belt.
For example, in a related art, a bias is applied between the belt and the cleaning member to thereby remove the developer from the belt. Further, by focusing attention on the fact that the state of such as the surface of the belt changes due to the aging deterioration and fouling, the voltage between a back roller disposed on the back side of the belt and a first obverse member disposed on the obverse side of the belt is subjected to constant current control so as to maintain satisfactory cleaning performance irrespective of the state of the belt. Additionally, for the purpose of this constant current control, the magnitude of the electric current flowing between the back roller and the first obverse member is measured, and the magnitude of the voltage between the back roller and the first obverse member is changed on the basis of the measured magnitude of the current.
However, according to studies conducted by the present inventors, it has become clear that even if the current is constant, the cleaning performance does not necessarily become consistent and an optimum cleaning bias changes due to changes in the environment and due to the aging and use of the cleaning member and the belt. As a method for detecting these changes, a method of estimating these changes on the basis of a resistance value at the time of the cleaning as been proposed. However, it is necessary to accurately measure the resistance value of the belt. Specifically, it is essential to accurately detect the current through the belt. However, it has also become clear that if the current is measured while a predetermined voltage is being applied between the cleaning member and the belt, the current varies based on the measuring position of the belt. Specifically, a bias is applied between the belt and the cleaning member, and a bias is also applied between the belt and each of a process unit and a transfer member, each disposed opposite the belt to effect image formation. These biases form a charged state corresponding to the biases applied to the belt surface, and this charged state affects the magnitude of the current flowing between the cleaning member and the belt during cleaning. Additionally, a similar problem occurs not only in the belt for transporting the recording sheet but also in an intermediate transfer belt for temporarily holding a toner image.